Recently, the development of new renewable energy has become more important and interested due to the serious environmental pollution and the lack of fossil fuel. Among the new renewable energy, a solar cell is spotlighted as an energy source for solving the future energy problem because it rarely causes environmental pollution and has the semi-permanent life span and there exists infinite resources for the solar cell.
Solar cells may be defined as devices to convert light energy into electric energy by using a photovoltaic effect of generating electrons when light is incident onto a P-N junction diode. The solar cell may be classified into a silicon solar cell, a compound semiconductor solar cell mainly including a group I-III-VI compound or a group III-V compound, a dye-sensitized solar cell, and an organic solar cell according to materials constituting the junction diode.
A solar cell made from CIGS (CuInGaSe), which is one of group I-III-VI Chal-copyrite-based compound semiconductors, represents superior light absorption, higher photoelectric conversion efficiency with a thin thickness, and superior electro-optic stability, so the CIGS solar cell is spotlighted as a substitute for a conventional silicon solar cell.
In general, the efficiency of the CIGS solar cell can be improved by diffusing sodium into a light absorbing layer. Meanwhile, according to the CIGS solar cell of the related art, the sodium may not be sufficiently supplied to the light absorbing layer or the supply of the sodium to the light absorbing layer may not be readily controlled.